The effects of alterations in photoperiod on ovarian development and spawning time in the rainbow trout (Salmo gairdneri)

12 month photoperiod cycle, normal yearly cycles compressed into 9 and 6 months, and a constant 12 hour light/12 hour dark regime and the effects of these experimental photoperiod regimes on the times of spawning of both male and female rainbow trout. Spawning was advanced by 6 weeks under the 9 months photoperiod regime, and by 12 weeks under the 6 month regime in comparison with the normal cycle. The levels of cestradiol 17p, calcium and phosphoprotein demonstrated under the simulated normal yearly cycle were markedly altered in female fish under the other photoperiod regimes and in each case coincided with the alterations in spawning time, thus indicating the involvement of these materials in the control of the processes of vitellogenesis and spawning. These results indicate that photoperiod is the major environmental factor in the control of the sequence of endocrine and other physiological changes which ultimately lead to spawning in salmonids and suggest that photoperiod may act by modifying an inherent reproductive rhythm.